Free up the IPI slot used by IPI_STOP_HARD.

[neel]

Summary

Change the numeric value of IPI_STOP_HARD so it doesn't occupy a valid IPI
slot. This can be done because IPI_STOP_HARD is actually delivered via NMI.

Test Plan

Enter debugger using 'sysctl debug.kdb.enter' which in turn uses IPI_STOP_HARD
to stop other cpus on amd64 and i386 systems.

[kib]

Does IPI_STOP_HARD value must fit into 8 bits ? It is not the same as the spurious vector, which should be innocent as well, but slightly confusing.

[neel]

In D1983#3, @kostikbel wrote:

Does IPI_STOP_HARD value must fit into 8 bits ? It is not the same as the spurious vector, which should be innocent as well, but slightly confusing.

There were two restrictions when choosing the value for the IPI_STOP_HARD value:

• 'native_lapic_ipi_vectored()' wants the vector to be less than 256
• 'xen_ipis[]' requires the vector to be greater than or equal to 243 (APIC_IPI_INTS)

[kib]

In D1983#6, @neel wrote:

In D1983#3, @kostikbel wrote:

Does IPI_STOP_HARD value must fit into 8 bits ? It is not the same as the spurious vector, which should be innocent as well, but slightly confusing.

There were two restrictions when choosing the value for the IPI_STOP_HARD value:

• 'native_lapic_ipi_vectored()' wants the vector to be less than 256

So if we change the assertion in native_lapic_ipi_vectored(), the IPI_STOP_HARD value could be moved outside the [0,255] range ? I mean, explicitly allow a value in the valid range OR IPI_STOP_HARD, in the assertion.

I do not insist, as I already indicated, I am fine with the patch as is.

• 'xen_ipis[]' requires the vector to be greater than or equal to 243 (APIC_IPI_INTS)

[neel]

In D1983#7, @kostikbel wrote:

In D1983#6, @neel wrote:

In D1983#3, @kostikbel wrote:

Does IPI_STOP_HARD value must fit into 8 bits ? It is not the same as the spurious vector, which should be innocent as well, but slightly confusing.

There were two restrictions when choosing the value for the IPI_STOP_HARD value:

• 'native_lapic_ipi_vectored()' wants the vector to be less than 256

So if we change the assertion in native_lapic_ipi_vectored(), the IPI_STOP_HARD value could be moved outside the [0,255] range ? I mean, explicitly allow a value in the valid range OR IPI_STOP_HARD, in the assertion.

Yup, that would have worked as well.

I do not insist, as I already indicated, I am fine with the patch as is.

• 'xen_ipis[]' requires the vector to be greater than or equal to 243 (APIC_IPI_INTS)

[neel]

Closed by commit rS279468 (authored by @neel).